The inclusion of hydrazine compounds for various purposes in silver halide emulsion layers has been described, for example, by Mees on page 281 of The Theory of the Photoqraphic Process, Third Edition (1966), in Research Disclosure, 23510 (November, 1983), and in U.S. Pat. Nos. 4,224,401, 4,243,739, 4,272,614, 4,323,643, 4,385,108 and 4,268,969. Particularly, hydrazine compounds are used in combination with internal latent image type silver halides to form direct positive images, and in combination with surface latent image type silver halides to achieve high speed and negative images having high contrast.
When hydrazine compounds are used in silver halide photographic materials for such purposes, they are preferably rendered fast to diffusion, which is to say water insoluble. Thus, the fast to diffusion hydrazine compounds do not dissolve out of the photographic material into the processing baths such that the baths do not become contaminated thereby, nor do they have any adverse effect on adjacent emulsion layers within the photographic material.
However, it is difficult to disperse such water insoluble hydrophobic hydrazine compounds in a hydrophilic silver halide emulsion in a stable manner.
Conventional methods used to disperse hydrophobic photographic additives in silver halide emulsions include methods wherein the additives are dissolved in a water miscible organic solvent such as methanol and then added to the emulsion, with the joint use of a surfactant, as required; methods wherein the additive is dissolved in a high boiling point organic solvent such as dibutyl phthalate or tricresyl phosphate and then agitated with a hydrophilic colloid to disperse the additives therein, with the conjoint use of a surfactant, as required, and wherein the resulting dispersion is then mixed with an emulsion; and methods wherein the additive is loaded onto a loadable polymer latex and then mixed with the emulsion, as disclosed, for example, in JP-B-58-35214 (the term "JP-B" as used herein means an "examined Japanese patent publication").
Other methods include those wherein, as disclosed in JP-A-61-34538, the aggregation and crystallization of the hydrazine compound in the coating liquid is prevented by mixing an essentially water insoluble hydrazine compound with the water insoluble organic compound to reduce the melting point of the mixture (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, when hydrophobic hydrazine compounds are dissolved in a water miscible organic solvent and added to a silver halide emulsion, the hydrazine compound coagulates and crystallizes in the coating liquid following addition and prior to coating. This crystallization cannot be prevented even with the use of large quantities of surfactants. Consequently, the hydrazine compound does not have its intended effect on the silver halide grains.
Furthermore, high boiling point organic solvents are effective for the dispersion of substances such as dye forming couplers and anti-color mixing agents which react with a diffusible active species (the oxidant of a color developing agent) formed during processing in color photography. However, high boiling point organic solvents are in appropriate for the diffusion transport of substances such as the hydrazine compounds which must undergo direct reaction with the non-diffusible silver halide grains in order to provide its intended effect. That is to say, the hydrazine compound is inevitably entrained in the dispersion when an amount of dispersing agent (high boiling point organic solvent) sufficient to provide a stable dispersion of the hydrophobic hydrazine compounds is used such that the hydrazine compound is unable to undergo a direct reaction with the silver halide grains.
Furthermore, methods wherein an essentially water insoluble organic compound is mixed with a hydrazine compound to reduce the melting point of the mixture is only effective for a period of about 6 hours after the coating liquid has been prepared. This method is effective as compared to systems in which the melting point has not been reduced, but the effect is inadequate over longer periods of time. Furthermore, it is difficult to completely prevent precipitation and coagulation when the hydrazine compound is used in a large amount (i.e., an amount of 1.5.times.10.sup.-3 mol or more per mol of silver). Moreover, there is a further disadvantage in that the presence of a low boiling point organic solvent causes adverse effects on the coating properties such as filter blockage at coating and ununiform coating.